A respiration mode APRV (Airway Pressure Release Ventilation), which is, similarly to the CPAP mode, a mode with a continuous positive airway pressure (CPAP), is known from practice. Contrary to the CPAP respiration mode, an upper pressure Phigh is applied in the APRV mode. This pressure Phigh is regularly lowered for a short period of time to a lower pressure level Plow this pressure reduction being called “pressure release.” The patient being respirated by means of APRV can breathe spontaneously at any time, but synchronization of the pressure release with the respiratory activity of the patient is not performed. It is typical of the APRV mode that a duration Thigh, during which the pressure Phigh is applied, is usually longer than a duration Tlow, during which the lower pressure Plow is present. FIG. 1 shows an example of respiration in the APRV mode in a simulation.
During respiration in the APRV mode, the mean respiration pressure is maintained at a comparatively high level, which leads to improved oxygenation. The elimination of CO2 is supported by the pressure release. Due to the short duration Tlow and the short duration of the pressure release, removal of air from the lungs to the extent that alveoli could collapse and could not participate in the ventilation any longer is prevented from occurring. The duration Tlow is set such that complete breathing out is prevented from occurring. It can be recognized from FIG. 1B that the pressure release is terminated before the patient flow has risen to 0 L/minute (see the times t=5.5 sec; t=13 sec; t=21.5 sec in FIG. 1B).
A new set value for APRV ventilation, which is circumscribed as “optimal flow termination based on a percentage of peak expiratory flow,” is proposed in the patent application US 2006/0174884 A1 of Nader M. Habashi. This set value will hereinafter be called % PEF (Peak Expiratory Flow). The parameter % PEF is set in percentage of a maximum expiratory flow or Peak Expiratory Flow (PEF). The pressure release is terminated when the instantaneous expiratory flow of the patient relative to the maximum expiration flow has dropped below the percentage set as % PEF (see for example FIG. 3). As a result, the algorithm automatically adapts the duration of pressure release to changes in the lungs. According to the above-mentioned US 2006/0174884 A1, the end of the duration Tlow shall be placed at the moment at which % PEF, which may be in a setting range between 25% and 50% of PEF (maximum expiratory flow), reaches or equals the set % PEF value. The expiratory gas flow is monitored for this purpose, PEF is determined, and the pressure release phase is interrupted when the expiratory gas flow drops below the preset percentage value % PEF.
If we assume in an example that % PEF is set at 50%, the pressure release is terminated in the APRV mode when the measured expiratory patient flow has dropped to 50% of the maximum value thereof. The expiration valve must be closed and an inspiration flow must be applied at this moment in order to achieve a pressure rise in the respiration system. However, it is known from practice that a valve always closes or opens with a time delay. In addition, system delays can be observed during the processing and transmission of information, e.g., a signal to the expiration valve, which indicates that the actual value has dropped below the set % PEF value. These delays lead, in their sum, to the lungs being able to be emptied further during the time that is due to these delays. The actual closing value of the expiration valve could thus be markedly below this value in the case in which the set value is 50% of % PEF. Likewise, when % PEF is set at 25%, a real closing value can be markedly below this 25%. However, since any random emptying is not allowed to take place for the above-mentioned reasons, the use of this % PEF criterion or parameter implies health hazards for the patient being respirated.